This type of conventional converters include a converter where a plurality of small-capacity DC/DC converters are connected in parallel, and are operated in parallel with shifting each operating phase of these converters in order to compose a single DC/DC converter (see Patent Document 1, for example).
Further, known converters include a converter where a DC-DC converter is operated synchronously with the resonance of a coil capacitor circuit containing the inductance of a transformer of the DC/DC converter by detecting the current applied to the transformer or the voltage of the node between a switching device and the transformer to detect the time point when the current becomes 0 A or the time point when the voltage becomes 0 V (see Patent Document 2, for example).
Furthermore, there is known a discharge lamp lighting apparatus where a discharge lamp lighting device is constructed by using a DC/DC converter resonating as in the case of the above Patent Document 2, and the power output to the discharge lamp is controlled by changing the drive frequency thereof by means of PFM (Pulse Frequency Modulation) by using the resonance produced by the leakage reactor of a transformer and a capacitor (see Patent Document 3, for example).
Moreover, a discharge lamp lighting device developed from the idea of the Patent Document 3 has conventionally been known, in which a DC/DC converter producing a resonance that is PMF-controlled as in the case of the above Patent Document 2 is employed, and further in the state just before the discharge lamp lights, the control by the resonance is released to control the device by the usual operation of the DC/DC converter (see Patent Document 4, for example).
Patent Document 1: JP-A-2000-012273
Patent Document 2: JP-A-1-114365
Patent Document 3: JP-A-2002-117995
Patent Document 4: JP-A-2003-059688
In the conventional device disclosed in the above-mentioned Patent Document 1, since a PWM waveform is generated by using a single triangular wave generator, two DC/DC converters shifted 180° from each other in phase (a multiphase DC/DC converter of two phases) can be easily constructed, ripple can be lowered, the efficiency thereof is high, and the noise thereof is low. However, because both the transformers thereof are driven at the same frequency and with the same duty ratio, both the transformers cannot simultaneously resonate (the resonances of individual transformers are different from each other according to their element variations), and besides, the conventional device cannot be developed to a resonant DC/DC converter achieving efficiency improvement and noise reduction
In the case of the conventional device disclosed in the Patent Document 2, the device includes a single DC/DC converter having a simple structure, also with high efficiency and low noise. However, since the device has a single configuration, its ripple is large, and its filter has to be reinforced to reduce the ripple. Additionally, because a resonance is produced by the inductance of a transformer and a capacitor, if a plurality of devices are simply operated in parallel, their frequencies tend to be different from each other, and the devices cannot be adjusted in phase.
Further, in the case of the conventional device disclosed in Patent Document 3, a discharge lamp lighting device is composed of a single DC/DC converter, and a resonant DC/DC converter is employed to reduce the switching loss. However, since a leakage reactor is used, its inductance becomes smaller than the inductance of the primary winding of the transformer, and the capacitor for resonance cannot but become large. Moreover, because of the single DC/DC converter, the filter for reducing ripple cannot but become large. Additionally, the PFM control thereof becomes more complex in the circuit structure than a PFM control controlling the ON-time of a switching device by using a triangular wave, and therefore, is not suitable for a simple small ballast.
Furthermore, in the case of the conventional device disclosed in the Patent Document 4, since a lighting device is composed of a single DC/DC converter as in the case of the device disclosed in the above third patent document, the filter for reducing ripple cannot but be designed large. Besides, even when a high power is introduced immediately after lighting, as well as at the timing before lighting, as shown in this Document, the release of control by the resonance may be required.
In addition, in order to increase the efficiency of a DC/DC converter and reduce noise generation, the idea of combining the devices disclosed in the above first and second paten documents is seemingly easy; however, development to the resonant operation from the device disclosed in the above first patent document cannot be expected. Even if independent DC/DC converters are operated in parallel with the devices as disclosed in the above second patent document, the operating frequencies is different from each other, and therefore, the advantage of the parallel operation cannot be effected.
The present invention has been made to solve the above-mentioned problems. Objects of the present invention are to provide a DC/DC converter device capable of causing DC/DC converters each having an individual resonant frequency because of their characteristic variations or the like to operate at the same frequency shifted in phase and making the best use of the advantages of resonant operation and parallel operation, and to provide a discharge lamp lighting device using the DC/DC converter device.